The present invention relates to an optical exposure apparatus of a scan-exposure system for transferring a pattern image formed on a reticle onto a wafer as the reticle and the wafer are relatively moved, and its exposing method, and more particularly to improvement of the reduction of a DOF (Depth of Focus) of a projection lens, which is caused by deformation of the reticle and upper and lower movement of the reticle at the time of scanning exposure.
Generally, in a lithography process of a semiconductor manufacture, an optical stepper of a reduction projection type is used. In the current optical stepper, the minimal pattern pitch of a device can be formed to be a quarter micron.
In the optical stepper, RES (resolution) of the pattern pitch of the quarter micron was achieved by highly enlarging a numerical aperture of the projection lens and shortening an optical exposure wave length (.lambda.) as shown in the following equation (1). EQU RES=k.sub.1.multidot.(.lambda./NA) (1) EQU DOF=.+-.k.sub.2.multidot.(.lambda./(NA).sup.2) (2)
wherein each of k.sub.1 and k.sub.2 is a constant, and NA is a numerical aperture of the projection lens.
As shown in equation (2), if the numerical aperture NA of the projection lens is highly enlarged and the optical exposure wave length is shortened, the DOF of the projection lens is lowered. This causes a so-called defective resolution in which a pattern image formed on a reticle becomes dim on the exposure surface of a wafer. In order to solve this problem, it is required that the projection image from the reticle and the exposure surface of the wafer be focused on each other with high accuracy.
FIG. 1 shows a schematic structure of the conventional optical stepper to improve the reduction of DOF. Specifically, for example, a reticle 3 is irradiated with light from a light source 1 through a condenser lens 2. A pattern image formed on the reticle 3 is reduced by a projection lens 4 so as to be projected on a wafer 5. As a result, photoresist on the surface of the wafer 5 is exposed by the projection image (reduced pattern image) from the reticle 3. At this time, the position of the exposure surface of the wafer 5 is measured by a wafer position measuring device 6. To focus the projection image from the reticle 3 on the exposure surface of the wafer 5 with high accuracy, the position of the wafer 5 in the direction of an optical axis (Z-axis) is controlled in accordance with the measuring result of the wafer position measuring device 6. In other words, the wafer position measuring device 6 comprises an LED 6a and a PSD (position sensitive detector) 6b. The driving system (not shown) in the Z-axial direction of the wafer 5 is controlled such that the output of PSD 6b is always set to "0."
However, by the above-mentioned wafer position measuring device 6, there can be obtained only data of the position of the exposure surface of the wafer 5 in the Z-axial direction. In other words, by the measuring device 6, there can not be obtained data of the position of the pattern image forming surface on the reticle 3 in the Z-axial direction, that is, one of causes of reducing DOF. In the conventional optical stepper, there was used a so-called step and repeat system. Specifically, the reticle 3 is mounted on a fixing state (reticle stage) 3', the position of a movable stage (wafer stage) 5' where the wafer 5 is mounted in each of X- and Y-axial directions is changed every time when each exposure is performed. In the optical stepper of this type, the deformation (curve and tilt) of the reticle 3 and the upper and lower movement of the reticle 3 at the time of exposure can be ignored. For this reason, it was unnecessary to obtain data of the position of the pattern image forming surface on the reticle 3 in the Z-axial direction.
However, in order to make it possible to decrease the diameter of the projection lens, there has been recently developed a so-called optical exposure apparatus of a scan exposure type in which exposure is performed as the reticle and the wafer are relatively moved. In this type of the apparatus, the deformation of the reticle and the upper and lower movement of the reticle during the scan exposure have a great influence on the reduction of DOF. For example, if the amount of deformation in the Z-axial direction at the pattern image forming surface on the reticle is .DELTA.Zm, the change .DELTA.Zw of a focal position at the exposure surface of the wafer can be expressed by the following equation (3). EQU .DELTA.Zw=.DELTA.Zm.multidot.R.sup.2 (3)
wherein R is a reduction ratio of the projection lens.
More specifically, if the amount of deformation of the reticle .DELTA.Zm is 2 .mu.m, the change .DELTA.Zw of the focal position at the exposure surface of the wafer is about 0. 13 .mu.m in the optical exposure apparatus of the scan exposure type whose reduction ratio of the projection lens is 1/4. This value cannot be ignored in the lithography process of the memory device in which higher integration and higher density are expected. In other words, the reduction of DOF in the optical exposure apparatus will become more and more serious in improving the integration and the density of the device.